"What can be done" is essentially impossible to answer without knowing which kind of things you like to do. I once spent a perfectly enjoyable layover in Frankfurt by taking a local train to the central station and looking at trains there until I had to take a train back -- but statistically that's not likely to fit whatever your idea of an enjoyable time is.
– Henning MakholmMar 28 at 20:28

2 Answers
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You're free to leave the airport during your layover. Of course you need to pass through the Schengen external border to do so, but as a Brazilian you're allowed visa-free short visits to the Schengen area, and "to pass time until my connecting flight leaves" is a perfectly valid premise for doing so.

Nobody's stopping people who leave the airport to demand proof that they don't have an onwards ticket.

Your hold baggage will likely be checked through to your destination in Brazil, so you won't get to take that with you into town. Though if you want to, you can probably get the check-in agent in Dublin to "short check" it. They're sometimes reluctant to do so, in order not to encourage "hidden-city ticketing", but since there's no reasonable way a combined ticket Dublin-Frankfurt-Brazil can be cheaper than just Dublin-Frankfurt, that's not really a risk with you.

On top of that, I'll have already been to Germany a few days before, so I guess I won't need to pass trough the interview again, correct?
– lord2701Mar 28 at 20:40

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@lord2701 There is no mandatory interview or vocal exchange at the immigration checkpoint. You are allowed to enter the Schengen area without a visa and without any particular 'cause' and very likely, the immigration officer will simply stamp you in without asking you a single question. If he wants to know something, he will ask you independent of your previous recent stay in Germany.
– Tor-Einar JarnbjoMar 28 at 20:48

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@lord2701: In principle all third-country national entering the Schengen area are supposed to be subjected to the same class of "thorough check" each time. This check is likely to be quite brief in your case, as border guards would want to focus their efforts on travelers with more risky profiles.
– Henning MakholmMar 28 at 20:48

You could get off at "Konstablerwache" walk leisurely down the Zeil (main shopping drag) to "Hauptwache", make your way past the "Paulskirche" and "Roemer" down to the river. Cross it using the the "Eisener Steg" and head into to "Sachsenhausen". Have some local "Apfelwein" (hard cider) and if you are are really adventurous "Handkaese" https://en.wikipedia.org/wiki/Handk%C3%A4se. Walk a bit along the river and come back across to the main station and get back to the airport.

That's about 45 minute of walk time with plenty of shops, restaurant, historical buildings/culture, and museums along the way to tailor to your specific preferences.